Tesla Model X SUV delayed again: Three possible reasons

Electric-car fans and Tesla owners alike are eagerly awaiting the debut of the Tesla Model X electric SUV, now scheduled to go into production roughly six months from now.

Like every Tesla, the Model X has been delayed a number of times--but there's been little speculation about the reasons for the delays.

Green Car Reports asked Tesla Motors to spell out what had led to the delays, and whether the car was still on track for a Q3 launch.

"We continue to develop Model X and prepare for production," wrote Khobi Brooklyn, Tesla's global communications director.

"We are currently testing our Model X beta vehicles and on track for a Q3 launch."

In other words: "Yes, it's going to launch in Q3, and we're not going to tell you anything about why it was late."

And yet, the final production version of the Model X hasn't been shown in public, even as the first pre-production prototypes would normally be rolling off the lines.

So we reached out to a number of sources, including those close to some of Tesla's parts suppliers, to ask why the Model X was delayed.

None of them would talk on the record--so what follows is thus largely speculation, though we'd suggest it's informed speculation.

For many, that will dilute its credibility. Still, we thought our various discussion would be worth summarizing.

Here are the three reasons we think the Model X schedule lagged significantly from its originally announced launch date of late 2013.

(1) RANGE RATINGS

EPA range ratings for the various Tesla Model S versions range from 208 miles (with the 60-kilowatt-hour battery pack) to 270 miles (the 85-kWh "D" all-wheel-drive variant).

The Model X crossover utility will have all-wheel drive as standard equipment--but it's larger and heavier, and has a greater cross-sectional area, than the lower, lighter, sleeker Model S.

A 60-kWh version could well be rated under 200 miles, likely making it a non-starter at a price of $70,000 or higher. The 85-kWh Model X would likely hit about 225 miles of range.

But we understand Tesla had really counted on replacing the door mirrors with video cameras to boost the range to around 250 miles.

Removing the door mirrors could add a significant boost to the range at highway speeds (where overcoming wind resistance consumes the majority of the energy).

But legal video mirrors aren't likely to happen soon, with the National Highway Traffic Safety Administration (NHTSA) having to study the issue, then issue proposed rules, wait for comments on them, and then put them into effect.

Even under the best of circumstances, that process almost surely will not happen by the third quarter of this year.

That means that Tesla is likely working furiously to get every single additional mile of rated range out of the Model X.

(2) STRUCTURAL IMPACT OF FALCON DOORS

One of the most striking features of the Model X is its falcon doors, which are hinged along the length of the center roof spine, and articulate as they rise to keep their width largely within the footprint of the car.

Tesla CEO Elon Musk said during an earnings call that the company "had learned a lot about door seals," and our sources tell us the sealing challenges are now solved.

Side impact ratings

There may remain two issues: Side-impact protection, and the effect of the complicated torsion springs on the aluminum roof structure.

Tesla buyers will likely expect the Model X to get the same top crash-safety ratings as the Model S in every category, including side impact.

That means the falcon doors themselves must contain strong beams that interlock into the rest of the body structure when the door is closed, to protect occupants in a side impact.

Such reinforcements can impinge on interior room, and significantly increase the weight of the doors.

Torsion springs for falcon doors

Which leads to the second challenge: anchoring the rotary door hinge mechanism for what is likely a fairly heavy door (since it contains a substantial chunk of roof too).

While aluminum is light and strong, its metallurgical characteristics differ from those of the heavier high-strength steel alloys now used in the latest vehicle structures.

Aluminum is best suited when loads are spread over a wide area--but it is vulnerable to shearing or stretching forces exerted on a small area.

The mounts for the large torsion springs of the falcon doors must thus spread their loads over a wide area, which may be a challenge given the narrowness of the roof spine shown in the concept versions of the Model X.

The roof structure cannot be vulnerable to warping over time, which is why Tesla has reportedly tested a number of high-strength alloys.

One of our sources suggests that the company could end up having to use an expensive titanium alloy to get the proper durability for the door-hinge mountings.

(3) TOWING CAPABILITY

The final challenge for the Tesla Model X is in providing the towing capability not currently offered by any plug-in electric vehicle.

Large SUVs are often used to tow anything from boats to horse trailers.

While the Model X will have nothing like the towing capacity of a heavy-duty pickup truck, its buyers might reasonably expect it to be able to tow a trailer with a couple of motorcycles or personal watercraft.

But sustaining the maximum output of electric motors in continuous use puts huge thermal loads on them--just as racing does.

The power of electric motors is often quoted at two levels: sustained and peak outputs. Towing requires high output to be sustained for miles or hours, which presents a huge cooling challenge.

Spreading the load over two motors helps--one source suggested that was why Tesla eliminated the single-motor Model X a couple of years ago.

But even the glycol-based coolant system in the Model S P85D may not suffice to protect the Model X motors while towing at capacity, especially if going from, say, San Francisco to Reno, a 5,000-foot increase in elevation.

Tesla may now be testing a thermal-conditioning system that uses refrigerant, like an air-conditioning system that actively removes heat from the coolant rather than simply shedding it via radiators.

In the end, Tesla has a track record of solving engineering challenges that conventional wisdom suggests are close to impossible.

And automotive engineers as a class are expert at identifying problems, analyzing their root causes, and then designing and testing solutions that solve them.

The big worry for more than one of our sources is this: Has the Model X team managed to solve all of these problems to the point that the electric SUV can go into mass production six month hence?

"I personally think we are 18-24 months away from a general Model X release right now," said the most pessimistic of our sources.

"I am just not seeing a company ramping up tooling and equipment to deliver a new vehicle in less than four or five months from now."

Tesla affirmed just last week, however, that in fact it is "on track for a Q3 launch."

The company is scheduled to release its fourth-quarter and full-year earnings this Wednesday after the stock market closes.

It may offer further details on the Model X schedule at that time.